Ronaldo joined the Red Devils from Sporting Lisbon back in 2003 before making the move to Real Madrid six years later, and his time at Old Trafford was a massive success under father figure manager Sir Alex Ferguson.
In the latest of his Nike video series - 'Chapter 4: Forged for Greatness' - Ronaldo recalled how he swapped the number 28 shirt for the number 7 at the behest of Ferguson and how the English club allowed him to flourish.
Ronaldo's new range of boots - the Mercurial Superfly - have been emblazoned with the Roman numerals XXVIII with the VII highlighted to depict his move from 28 to 7.
“I remember when I left Lisbon for England, Alex Ferguson said to me 'listen, I want you to take the No 7'," Ronaldo said.
“I was surprised because I knew the guys who wore that shirt. I accepted the challenge, it was great.
“It was the beginning of my success in football. It was probably one of the most beautiful moments in my career so far.
“Alex Ferguson was a father for me in football. And I learned many things, not just from the coach but the players.
“I improved not just my skills but my body too.
“In the five years that I was (at United) I learned the art of football, we won many things together, it was a dream for me."
Watch the video below, thanks to Nike Football: